A Royal Navy serviceman is being held on suspicion of murder after an officer was killed and a second was seriously injured in a shooting incident on board a nuclear-powered submarine.

A group of children had just left HMS Astute, which is docked in Southampton on a goodwill visit, and a party of dignitaries was being shown around when the incident happened.

Among the visitors on board was the leader of Southampton city council, Royston Smith, who last night said he had helped wrestle the person allegedly behind the shooting to the floor.

The Ministry of Defence said reports that the shooting followed a row over use of the submarine's five toilets during the civic visit would be investigated.

A police spokesman said: "Hampshire police were called by their Ministry of Defence colleagues at 12.12pm today and are liaising with them to establish the exact circumstances of the incident." An MoD spokesman said: "Two Royal Navy personnel have been involved in a firearms incident at Southampton docks where HMS Astute is alongside. Sadly, one has now died as a result of his injuries.

"The Royal Navy is now attempting to inform their families as a matter of urgency. A third Royal Navy serviceman has been arrested by Hampshire constabulary and is now in custody."

Last night, there were calls for an inquiry to establish if the public was put in danger, and campaigners said the incident should prompt a rethink about the wisdom of using nuclear submarines.

The police and navy emphasised that submarine security was not breached and were convinced it had not been a terrorist incident. A navy spokesman said: "There is no nuclear incident ."

It is understood that the two victims were male officers and that the weapon used was an SA80 rifle.

Smith, 46, described how the alleged gunman burst into the control room in body armour and camouflage gear brandishing the rifle. He said he had heard six shots. "I heard a hiss which meant one had flown just passed me," he said. "I charged at him, pushed him against the wall and, at the same time got hold of his weapon. We had a tussle and I think I was shouting quite a lot.

"I have no idea why it happened. It is unprecedented for something like this to happen on one of our submarines."

He played down the idea there had been a row over the toilets.

Police said the second man hit by gunfire suffered "significant injuries" but described his condition as stable.

John Denham, the Labour MP for Southampton Itchen, expressed concern about security. He said: "I will be asking ministers to ensure this incident is fully explained."

Di McDonald, chair of the Nuclear Information Service, which promotes awareness of nuclear disarmament, said the incident ought to prompt a rethink on the value of confining men in such an unnatural environment.

Liam Fox, the defence secretary, said: "It is right and proper that a full police investigation is carried out."

Astute, which has a crew of 98, was the first of a new class of submarine. The visit to Southampton was billed as the first chance for members of the public outside Scotland and the north-west of England, where she was built, to see her. She was launched in 2007 and commissioned last year, only to hit the headlines when she grounded off Skye in October.

• This article was amended on 12 April 2011. The original caption referred to HMS Astute, pictured in 2001. This has been removed.